Concentration Fluctuation Intensity Research Articles

TURBULENT MIXING IN THE IMPINGEMENT ZONE OF DUAL OPPOSED FREE JETS AND OF THE NORMAL WALL-IMPINGING JET

Marker nephelometry has been used to study the concentration fields of two jet-mixing systems: (i) equal opposed turbulent round free jets impinging upon each other, and (ii) the turbulent round free jet impinging upon a plane wall normal to its axis. Attention is focussed upon the impingement or deflection zone. The fields of mean concentration, concentration fluctuation intensity, and concentration intermittency were measured. Two-point correlations and frequency spectra were determined at selected points. Integral spatial scales were estimated. In the case of the opposed jets, the extent of mixing between the two jet source fluids was mapped. The results are of interest in relation to chemical reactors and combustors and for applications in heat and mass transfer.

Diffusion of matter by a non-buoyant plume in grid-generated turbulence

The turbulent diffusion process is investigated for a continuous point source of a non-buoyant plume in grid-generated water turbulence. Two kinds of biplanar grids with a mesh length of 10 mm and 20 mm were used. The mesh Reynolds numbers were 1480 and 2970, respectively. The mean and fluctuating concentration fields of aqueous dye solution were measured by the light absorption method. Experimental results for both grids were compared.For both grids, the mean concentration radial profiles proved to have a similar Gaussian shape, and the mean concentration on the plume axis obeys the hyperbolic decay law well. These mean concentration profiles and their decay show an excellent agreement with the results deduced from the similarity analysis for the mean concentration field.Radial profiles of the fluctuation r.m.s. value and relative intensity (i.e. the ratio of the r.m.s. value to the mean concentration) were found also to be nearly similar, and the centreline r.m.s. value decays downstream as a hyperbola. The relative intensity on the centreline tends to increase slightly downstream. All experimental results obtained were much less scattered and more reliable than those reported earlier.The similarity for the concentration fluctuation intensity has been analysed using a thin-layer approximation. Also, an approximate analysis of the fluctuating concentration field is given by replacing the fluctuating concentration signal by a randomly spaced sequence of rectangular waves with various heights and widths.

On the relative dispersion of two particles in homogeneous stationary turbulence and the implications for the size of concentration fluctuations at large times

AbstractA model for the separation of two particles in homogeneous stationary turbulence is derived by considering the analogous process of one‐particle dispersion in inhomogeneous turbulence. Numerical simulations suggest that the intensity of concentration fluctuations tends to zero at large times.

Diffusion of Matter from a Continuous Point Source in Uniform Mean Shear Flows : 2nd Report, Characteristics of Concentration Fluctuation Intensity

Resultats experimentaux concernant l'intensite de fluctuation de concentration. Les profils horizontaux et verticaux des valeurs quadratiques moyennes de la fluctuation de concentration sont presque semblables

A study on the diffusion of matter from a continuous point source in uniform mean shear flows (2nd report, Characteristics of the concentration fluctuation intensity)

The turbulent diffusion process of dye solution has been examined experimentally in the case of a continuous point source in the water turbulent flows with three different uniform mean shear rates. In this paper, the experimental results of the concentration fluctuatin intensity are investigated. Already, the characteristics of the mean concentration field have been reported in ref. (1). The data show that the horizontal and vertical profiles of the r.m.s. values of the concentraiton fluctuation appear nearly similar and especially, vertical profiles skew to the side with the smaller flow velocity. The above characteristics are the same as the results of the mean concentration field. Further, by using the random rectangular wave model for the concentration fluctuating signal, an analysis for the concentration fluctuation intensity is made. The caluculated profiles expressed quite well the experimental results.

Ground-level concentration fluctuations from a buoyant and a non-buoyant source within a laboratory convectively mixed layer

Surface concentration and its fluctuations from plume dispersion under unstable conditions in a coastal environment are investigated using the model validation program field experimental data. The goal of this study is to better understand plume dispersion under such conditions. Procedures are described to derive the plume surface concentration from moving vehicle measurements. Convective boundary layer scalings are applied and cumulative density functions (CDF) are studied.The results indicate that the relative concentration fluctuation intensity (σc/C(y)) decreases with the normalized downwind distance (X) and that it is relatively small at the plume central line and largely increased at the plume edges, consistent with other field and laboratory results. The relation between σc/C(y) at the plume centerline (σc/C) and X for elevated sources can be described by σc/C=a+b/X. The crosswind plume spread (σy) is found to satisfy Deardorff and Willis's (J. Appl. Meteorol., 14 (1975) 1451) form of σy/h=a1X/(1+a2X)1/2 scaled with convective layer depth h. For elevated sources, the normalized crosswind integrated concentration (Cy) is found to satisfy a relation of Cy=16X-3/2, with Yaglom's (Izr. Atmos. Oceanic Phys., 8 (1972) 333) scaling rule on the free convective layer being applied.Empirical CDFs based on the gamma and the clipped probability density functions show agreements with the experimental CDFs, with the former being better than the latter when (c-C)/σc>0.5. A new clipped-gamma CDF form is proposed based on the analysis of the present data, showing a better agreement.We suggest that a parameter u0*(12-0.5h/L)1/3, with combined efforts of surface friction velocity (u0*), Monin–Obukhov stability length (L) and unstable boundary layer height (h), replace the convective velocity scale (w*) under weak convective conditions in a coastal environment.

Gas dispersion near a cubical model building. Part II. Concentration fluctuation measurements

A wind-tunnel study has been performed of concentration fluctuations in the near-wake region (1.0 ⩽ x/ H ⩽ 5.0) of a cubical model building in a simulated, neutrally stratified shear layer. Contaminants were released at a central roof vent for buildings with 0° and 45° orientations, and at a downwind roof vent for a building with 0° orientation. The log-normal concentration probability model was found appropriate for measurements in the building wake, and concentration fluctuation intensity was found to be reduced by the presence of the model building in an obstructed flow. A simple algorithm, based on the relation of the peak-to-mean concentration ratio to the local intensity, suggests an upper limit to the peak-to-mean concentration ratio near the ground centerline.

Local turbulence properties in flames from time-averaged Raman spectroscopy measurements

/Local fluctuations in temperature and composition in a turbulent flame are examined through their influence on Raman-scatter ing measurements of time-averaged flame properties. Using mean temperatures and number densities measured in a hydrogen diffusion flame, two approaches to investigating turbulence properties are examined. In the first, which is restricted to flame conditions where fluctuations are fairly small, the variation of measured number density with spectral resolution is used to determine values for particular correlations between fluctuations in number density and temperature. In the second approach, the Raman measurements are used to evaluate a particular model for concentration fluctuations. The shifting-equilibrium reaction model is combined with a clipped-Gaussi an probability density function for mixture fraction in order to predict time-averaged Raman signals. Comparisons with the Raman data determine particular radial distributions for the mean mixture fraction and for the intensity of concentration fluctuations. These distributions are then used to predict a variety of averaged flame properties, as well as the errors occurring in the time-averaged Raman measurements.

The turbulent concentration field of a methane jet

Detailed measurements of turbulent concentration parameters in a round free methane jet up to 70 diameters downstream from the jet source are described. The concentration changes were obtained using a Raman spectrometer designed for the measurement of a methane vibrational transition function, and processed using a photon correlator. Mean and fluctuating concentration levels are given, together with the concentration probability density distribution.A theory for interpreting the statistical aspects of the signal is presented and results are compared with other data from the literature on jets of uniform and non-uniform density. It is shown that the intensity of concentration fluctuations has an asymptotic value of 28·5% in the far field region of the jet. One interesting feature of the data is the deviation from Gaussian statistics along the jet centre-line.

Laser light-scattering measurements of particle concentration in a turbulent jet

A laser light-scattering technique has been used to study the relative particle concentration field in a round turbulent air jet. Measurements were made in the far field of a smoke-marked turbulent jet exhausting into a secondary air stream. Radial distributions of mean particle concentration, concentration fluctuation intensity and intermittency were measured at several streamwise locations. Concentration fluctuation power spectra and the micro- and integral scales of the concentration fluctuations were measured on the jet axis. The effects of ambiguity noise and noise due to optical path attenuation on the performance of the laser light-scattering system are discussed.

Mixing in the interaction zone of two free jets

AbstractMixing in the interaction zone of two round free jets with nozzle axes intersecting at half‐angles of 15, 30, and 45 deg. has been studied by the smoke scattered light technique. The fields of mean concentration and concentration fluctuation intensity were mapped with one jet marked with smoke, the other jet marked, and both jets marked. The correlation function between the concentration fluctuations associated with fluid of the two nozzle streams has been calculated from the results and is an important characteristic of the mixing field and a significant measure of molecular mixedness. A transport equation for the associated covariance function is derived, and the special features of the mathematical modeling problem for this class of mixing process are discussed. The implications of the results for processes involving chemical reactions are examined.

Concentration probability distribution and intermittence in turbulent jets

In a turbulent flow with mixing of the temperature and concentration, the velocities are random quantities; therefore, the appropriate probability distributions are necessary for averaging different relations. This requirement is especially strong when analyzing combustion processes, and the stabilization and propagation of flames in a turbulent flow. Under these conditions the mixing and chemical kinetics should be considered together, and since the chemical reaction rate has a nonlinear dependence on the temperature and concentration, the temperature and concentration probability distributions should be known in order to solve the problem. It is natural to attempt to obtain and solve the appropriate probability equations [1–3]. It is possible to derive these relations from the equations of motion, diffusion, and thermal conductivity [1, 2], but some assumptions are necessary in order to obtain a self-contained description. On the other hand (and this applies to the present paper), the required equation can be obtained from certain phenomenological considerations [3–6]. It follows from this equation that the concentration probability is distributed according to the normal law in regions without infermittency. The equation enables one to calculate the intensity of concentration fluctuations at the edge of a turbulent flow for a completely turbulent fluid.

Measurement of diffusion coefficients of meningococcal polysaccharide by optical mixing spectroscopy. I. A preliminary characterization on the aggregation of the group C polysaccharide.

AbstractThe angular distribution of scattered intensity and decay times of concentration fluctuations have been measured by means of digital photon counting and single‐clipped photon correlation for solutions of Group C meningococcal polysaccharides at 31°C. The z‐average diffusion coefficient 〈D〉z and its second moment 〈D2〉z have been determined from the time‐dependent correlation function using the cumulant expansion technique. Very low observed values of 〈D〉z and the tremendous width of the polydispersity index, which is the z‐average normalized variance, suggest a higher degree of aggregation than the monomer–dimer type self‐association at finite concentrations.

Turbulence measurements with hot-wire anemometry in non-homogeneous jets

The axial distributions of some turbulence quantities in round jets discharging into quiescent air have been measured. The primary purpose of the investigation was to establish the validity of a measurement technique using hot-wire anemometry previously described by the authors. The distribution of the intensity of axial velocity fluctuations has been measured in a jet of carbon dioxide, and the distributions of the covariance of axial velocity and concentration fluctuations, and of the intensity of concentration fluctuations, have been measured in a jet of argon. The measurements are compared with those made by Way and Libby and by Becker, Hottel and Williams, each using alternative techniques, and good agreement is found with the measurements reported by Way and Libby.

Ice Nucleus Concentrations at −20C During Convective Storms

Increases in surface ice nucleus concentrations at −20C (by a factor of 10 or higher) have been measured during precipitation from hailing and non-hailing convective storms; these increases are associated with the storm downdrafts. The intensity of the ice nucleus concentration fluctuations and the concentration at −20C are similar in both Alberta and Quebec storms. One of the seven convective storms for which measurements are presented was seeded with AgI released from an aircraft; the seeding material was subsequently detected at the surface. If the observed higher ice nucleus concentration in the downdraft mixes with a storm updraft of 10 m sec−1, simple calculations indicate that no dramatic change would occur in the ice content of the updraft: to produce 5 gm m−3 of ice by −20C, in such an updraft, 105 times the normal background concentration of ice nuclei would be required.

An analysis of clustering and ordering in multicomponent solid solutions—II fluctuations and kinetics

The free energy of a multicomponent substitutional solid solution is rederived on the basis of a more rigorous averaging procedure than the one used in I. The resulting expression is a quadratic form in the Fourier transforms of the concentration variables. Application of the Landau theory of fluctuations then leads to a set of formulas for the expectation values of concentration fluctuation intensities, which are the extension to multicomponents of the Clapp and Moss formulas for binary systems. Coupled discrete-space diffusion equations are derived and solved by diagonalization of the diffusion matrix. Principal directions in composition space and static and kinetic eigen vectors are defined. The effect of the positive definite mobility matrix is to rotate the directions of fastest and slowest initial concentration changes (kinetic directions) away from the principal free energy directions but in such a way that asymptotic directions are never crossed. Examples of ternary systems are given and asymptotic directions in quaternary systems are discussed qualitatively.

Concentration fluctuations and peak-to-mean concentration ratios in plumes from a ground-level continuous point source

Diffusion data collected by 63 incremental samplers during four short duration, continuous releases of 85Kr are used to examine concentration fluctuations and peak-to-mean concentration ratios in the atmosphere. Cumulative frequency distributions and the intensity of short-term concentrations are shown to be a function of the relative crosswind position within the mean plume. Peak-to-mean concentration ratios are shown as a function of relative crosswind position within the plume and the ratio of the durations of the mean and the peak (time ratio). The relationship between the intensity of concentration fluctuations and the relative crosswind position is similar to that reported for laboratory data. The relationship between peak-to-mean ratios and the time-ratio was found to differ from the simple power law relationship suggested by several workers. Previous estimates of the peak-to-mean/time ratio relationship for positions near the mean plume axis are shown to be too low.

Molecular mixing lengths for turbulent wakes.

Abstract : A simplified analysis is performed of the turbulent mixing of a trace chemical species with the bulk chemical species. The general relationship between the mixing on a molecular scale and the concentration fluctuation intensity of the trace species is given. This is then applied to a turbulent wake, to determine the axial length that it takes for ingested gas to become mixed. The results are sensitive to the assumed turbulent dissipation rate; giving values of the mixing length from the order of the wake radius to two orders of magnitude larger.

Concentration intermittency in jets

A refined analysis of the combustion process in a turbulent diffusion flame requires knowledge of at least three aspects of the concentration field of nozzle fluid: the mean concentration, the concentration-fluctuation intensity, and the intermittency factor. The last is defined as the probability of finding nozzle fluid at a point, and was the subject of this investigation. A cold model was used in which nozzle air, marked with an oilfog, discharged into clear air at the same temperature. The scattered-light technique was utilized for concentration detection. Both free and ducted round, turbulent jets were studied. With the latter, emphasis was on the regime in which recirculation occurs. The mechanism of recirculation was considerably illuminated by the results. The complement of the intermittency factor is interpreted as the probability distribution function of, for example, the lateral location of the jet boundary in zones without recirculation. Thus, it was possible to determine the statistical properties of the jet boundary, and also of the upstream edge of the recirculation eddy.